1. Field of the Invention
The present invention relates to curable compositions having long term moldability, particularly epoxy-based molding compounds exhibiting moisture resistance and long term moldability without staining the mold surface.
2. Brief Description of Related Technology
Epoxy resins are widely used in molding compounds for coating electrical and electronic devices. Such epoxy molding compounds used for encapsulation are generally prepared from a blend of an epoxy resin and phenol hardener, along with other ingredients including fillers, catalysts, flame-retardant materials, processing aids, and colorants. Epoxy resins in such molding compounds are traditionally diepoxides which include two epoxy groups per molecule, which are reacted with a co-reactant (cross-linking agent or hardener), such as an acid dianhydride, diamine, or diphenol oligomers. Diphenol oligomers, such as those derived from novolac phenols, cresol phenols and bisphenol A, are particularly preferred in the art as hardeners due to their high reliability.
An important feature of such epoxy molding compositions is their ability to mold within standard industry molding operations to provide an encapsulated electrical or electronic device which does not include any defects, whether functionally or cosmetically. For example, during the molding operation, the molding composition is subjected to increased temperatures to cause curing of the composition and encapsulate the electronic device. The composition should exhibit appropriate adherence to the surfaces of the electronic device to prevent delamination, should not include any voids, blisters, knitlines, etc., and should properly release from the mold equipment without causing any staining, flow marks, or the like.
The Joint Electron Devices Engineering Counsel (JEDEC) has set forth a series of test parameters for proper moldability for such molding compositions. JEDEC Level 1 testing requires molding materials to be exposed to 85° C. and 85% relative humidity for a period of 168 hours, after which time the molding materials are heated to a temperature of 260° C. The molding material is then analyzed for any voids or defects and for delamination.
It is recognized in the industry that stress reliever compounds such as polyphenylene oxide can be incorporated into such molding compositions to reduce package stress and improve interface adherence properties required for achieving the JEDEC Level 1 testing procedures. However, such polyphenylene oxide compounds may cause the molding equipment to develop mold stains, or may cause defects in the aesthetics of the molded article such as flow marks on the article, during prolonged use. For example, typical transfer molding operations may involve a single piece of molding equipment which is continuously charged with a molding composition in the form of a pellet. The pellet is heated and flowed or forced into a molding cavity to envelop an electronic substrate contained in the mold cavity and to cure, thereby encapsulating the substrate in a desired shape according to the shape of the mold. During prolonged use, the molding equipment is continuously charged with a substantial number of pellets, commonly referred to as “shots”. After prolonged use, mold stains may develop in the molding cavity and/or flow marks may appear on the molded articles, believed to be due to one or more compositions within the molding compound forming a streaking effect.
Accordingly, it would be desirable to provide molding compositions that exhibit long term moldability including interface adherence properties capable of meeting industry standards without adverse affects such as mold staining or flow marks.